The invention relates to a steering loop for missiles which are guided to a target by means of a seeker head. The steering loop has a seeker head, which determines the line of sight to a target by look angles with reference to missile-fixed pitch and yaw axes. The seeker head provides seeker signals. The seeker signals are applied to signal processing means. The signal processing means generate signals determining the motion of the missile. The steering loop, furthermore, contains steering means for guiding the missile to the target. The signals from the signal processing means are applied to these steering means.
Conventionally, the seeker head has an imaging optical system and a sensor. The imaging optical system has an optical axis. A control loop including the sensor causes the optical axis of the optical system to point towards a target detected by the sensor. Then this optical axis defines a "line of sight" to the target. The orientation of the line of sight relative to the missile can be defined by two "look angles" about a yaw axis and a pitch axis, respectively. The optical system with the sensor represents a "seeker".
The seeker head provides seeker head signals. Steering signals are generated in accordance with a steering law, the steering signals guiding the missile to the detected target. According to the steering law of "proportional navigation", for example, the steering signals are proportional to the angular rate of the line of sight in inertial space. The steering signals control the movements of the control surfaces. With proportional navigation, the steering system seeks to maintain the line-of-sight stationary in space. The control loop with the seeker as measuring element and the control surfaces (or the like) as actuator is called "steering loop", by which the missile is guided to the target.
A lateral acceleration of the missile is to be achieved by the movement of the control surfaces. To this end, the missile changes its angle of attack, i.e. the angle between the flight velocity vector and the longitudinal axis of the missile. By this change of the angle of attack, the look angles of the seeker head are changed. The missile changes its attitude in space relative to the substantially space-fixed line of sight.
The optical path of rays of the seeker passes through a window near the tip of the missile. This window determines the field of view of the seeker. For optical and aerodynamic reasons, this window is often provided sidewards at the tip of the missile. Thereby, the amount of the admissible look angles is limited. If the line of sight to the target leaves the field of view of the missile, the seeker head loses the target.
Examples of missiles having windows sidewise at the tip of the missile are shown in U.S. Pat. No. 4,717,822 and European patent application 0,482,353.